A great deal of effort has heretofore been made for attaining higher sensitivity of silver halide photographic light-sensitive materials. In a silver halide photographic emulsion, a sensitizing dye adsorbed to the surface of a silver halide grain absorbs light entered into a light-sensitive material and transmits the light energy to the silver halide grain, whereby light sensitivity can be obtained. Accordingly, in the spectral sensitization of silver halide, it is considered that the light energy transmitted to silver halide can be increased by increasing the light absorption factor per the unit grain surface area of a silver halide grain and thereby the spectral sensitivity can be elevated. The light absorption factor on the surface of a silver halide grain may be improved by increasing the amount of a spectral sensitizing dye adsorbed per the unit grain surface area.
However, the amount of a sensitizing dye adsorbed to the surface of a silver halide grain is limited and the dye chromophore cannot be adsorbed in excess of a single layer saturation adsorption (namely, one layer adsorption). Therefore, individual silver halide grains currently have a low absorption factor in terms of the quantum of incident light in the spectral sensitization region.
To solve these problems, many methods for attaining larger adsorption of a sensitizing dye than one layer absorption have; been proposed. For example, JP-A-2002-23294 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) describes related documents and patents in the paragraph of Background Art. Particularly, attempts are being recently made to combine a specific cationic dye and a specific anionic dye so as to attain high sensitivity by multilayer adsorption (see, for example, JP-A-10-239789, JP-A-10-171058 and EP-A-0985965).
However, the sensitivity level in these methods is not yet satisfied and the silver halide photographic light-sensitive material is demanded to have still higher sensitivity.
When a tabular silver halide grain having a high aspect ratio (hereinafter called a tabular grain) is used, by virtue of its large ratio of surface area to volume, a large amount of a sensitizing dye can be adsorbed to the surface and a higher color sensitization sensitivity can be obtained as the photographic property (see, for example, U.S. Pat. No. 5,494,789). The aspect ratio as used herein means a ratio of diameter to thickness of a tabular grain. The diameter of a tabular grain indicates a diameter of a circle having an area equal to the projected area of a grain when an emulsion is observed by a microscope or an electron microscope. The thickness is a distance between two parallel planes constituting a tabular grain. In this way, the tabular grain is advantageous for obtaining high color sensitization sensitivity. For attaining high sensitivity, selenium sensitization of a silver halide emulsion is also useful and many selenium compounds are known (see, for example, JP-A-4-109240). However, the sensitivity level in these methods is not yet satisfied and the silver halide photographic light-sensitive material is demanded to have still higher sensitivity.